1. Field of the Invention
This invention relates to a cold cathode fluorescent lighting system, more particularly to a stationary ion cold cathode fluorescent lighting system comprising a cold cathode fluorescent lamp with a radial D. C. electric field established therein to reduce the radial velocity of the ions therein to a virtual zero when touching the phosphors evenly coated on the inside surface of said lamp, and with an axial anti-equivalent D. C. electric field established between the two electrodes of said lamp so as to prevent a mercury accumulation on the cathodes and to lengthen life of said lamp.
2. Prior Art of the Invention
Referring to FIG. 1, the glass lamp 10 of a conventional fluorescent lighting system is mainly used for forming a closed space filled mainly with some inert gas e.g., argon (Ar) at a pressure about 0.3% atmosphere pressure and mercury (Hg); a layer 11 of phosphor layer e.g., zinc silicate (Zn2SiO4) on the inside surface of the lamp is used for illuminating purpose; both the two ends of the lamp are provided with tungsten filaments 121, 122 (also used for cathodes); one end of each of the filaments 121, 122 is electrically connected through an insulating end plug 123 to a starter 13; the other end thereof is electrically connected through the insulating end plug 123 to an A. C. power source 16 via a ballast 14 and a switch 15, respectively. Referring to FIG. 2, said starter 13 mainly comprises a parallel capacitor C and a neon lamp 130 provided therein with two electrodes 131, 132 which are unconnected electrically when the system is not operating, wherein the bimetal electrode 132 tends to bending toward making an electrical contact with the electrode 131 under thermal influence; the ballast 14 is an inductor comprising a solenoid with a soft magnetic core for coordinating with the A. C. electric power source 16 and the ballast 14 to control the current I13 of the starter and the current of the lamp 10 and safeguard the starter 13 and the lamp 10.
Referring again to FIG. 1 and FIG. 2, the operating sequence of said lighting system is as follows:
1. After the switch 15 is turned on, the bimetal electrode 132 bends and makes an electrical contact with the electrode 131 under the thermal influence of the neon discharge between said two electrodes, forcing a current I13 through and heating up said filaments 121, 122 so as to release massive thermionic electrons while the lamp 10 is still not conducting.
2. Once an electrical contact is made between the electrodes 131, 132 of the bimetal switch, the bimetal electrode 132 cools off and breaks away from the electrode 131, interrupting the current I13 in the starter 13, the filaments 121, 122 and the ballast 14.
3. At the interruption of the current I13, the ballast (inductor 14) induces a voltage of 1500 volts, wherein approximately 600 volts thereof is applied briefly between the two points a and b as shown in FIG. 1 which is too low to trigger the neon lamp 13 but is high enough to energize the thermionic electrons into an argon discharge with massive argon ions (Ar+2) and argon electrons ea− (a symbol for differentiating from the mercury counterpart) produced; said argon electrons ea− are energetic enough to force the mercury into a process of chain discharge with massive mercury ions (Hg+2) and mercury electrons (eh−) produced. Through the process of splitting and recombination, these particles form a plasma in the lamp 10 in equilibrium state with the mercury electrons (eh−) carrying an ultraviolet (UL) light of 2537 Å (10 exp–10 m) useful for phosphorescing purpose, while the argon electrons ea− carrying their characteristic UV light for ionizing the mercury.
When the said particles touch the phosphor layer 11 on the inside surface of the lamp 10, they affect the phosphors differently. The argon electrons ea− do not cause the phosphors to illuminate; the mercury electrons eh− force the phosphors to illuminate visible light (380 Å˜780 Å) according to Stokes Law and the following photoelectric quantum formula:
                                          Δ            ⁢                                                  ⁢            W                    =                      hc            λ                          ,                            (        1        )            wherein ΔW refers to the released energy, h refers to Planck constant (6.62517 exp (−34) j×sec), c refers to the light speed (3 exp 8 meter per second), λ refers to the wavelength of the phosphoresced light. Again referring to FIG. 1, since the production of the light by this system is initiated by the thermionic electrons from the hot cathodes in its lamp, this conventional lighting system is called “hot cathode fluorescent lighting system”, which is abbreviated as “HCFL”.
It is known that the phosphor layer 11 in the coating is of a crystal structure with the atoms fixed in the lattice, and is capable of phosphorescing visible light when suitably excited by some radiation (e.g., the said UV radiation of 2537 Å) as long as the crystal structure remains undisturbed. However, when the phosphor layer 11 is bombarded by the high energy mercury and argon ions, the atoms in the lattice can be dislodged easily, resulting in forming a non-luminescent, discharge absorbing amorphous layer on the phosphor layer 11 according to the following formula:S=C√{square root over (tI)},  (2)wherein I refers to the bombarding current in the lamp, t refers to the duration of time when the current is on, C refers to a constant describing the stability of the phosphor against damage by bombardment, S refers to the thickness of the amorphous layer.
The luminosity of the phosphor layer decreases with the thickness of the amorphous layer. The mercury embeds in the phosphor layer in an irreversible process. Therefore, the forming of an amorphous layer and the embedding of the mercury decrease the luminosity of the phosphor layer 11 according to the following formula:
                                                        B              t                                      B              0                                =                      exp            ⁡                          (                                                -                  a                                ⁢                                                                  ⁢                                  C                  ·                                      tI                                                              )                                      ,                            (        3        )            wherein Bt refers to the luminosity at time t, B0 refers to the initial luminosity, a refers to the light absorbing constant of the amorphous layer. Because a and C are constants, and I is roughly constant in operation, they can be combined for obtaining the Lehmann formula below:
                                                        B              t                                      B              0                                =                                                    exp                ⁡                                  (                                      -                                                                  t                        /                        υ                                                                              )                                            ⁢                                                          ⁢              υ                        =                          1              /                              (                                                      C                    2                                    ⁢                                      a                    2                                    ⁢                  I                                )                                                    ,                            (        4        )            which has been confirmed by Willi Lehmann in his report in J. Electrochem. Soc., 426 (February, 1983) and by Osamu Tada's report in J. Electrochem. Soc., 1366 vol. 131 No. 6 (June, 1984). Therefore, it can be determined that, the luminosity of the fluorescent lamp 10 decreases with time by the amorphous layer formed on the phosphor layer and the mercury embedding in the phosphor layer.
Regarding the liquid crystal display (LCD), especially for the portable type LCD, where the cold cathodes fluorescent lamp (abbreviated as “CCFL”) is used for backlighting the liquid crystal display, as shown in FIG. 3, which has the following differences in comparison with the structure of the “hot cathode fluorescent lighting system” shown in FIG. 1:                1. Without filaments in the lamp,        2. Without neon starter,        3. Without ballast, and        4. A self-contained high frequency A. C. power source 26 for the CCFL.        
In the case of a lighting system with a lamp of 3 mm×160 mm, as shown in FIG. 3, it requires to apply a voltage of 1600 volts at 55 kHz in frequency from A. C. power source 26 to its two cathodes 221, 222 for turning on the light.
For both the HCFL and the CCFL lighting systems, the ideal power source is to be with a waveform of zero crest factor. The system as shown in FIG. 1 can easily meet this requirement, as it is powered from an ideal mains of A. C. sine wave power source of the frequency 60 Hz. But due to the component spread, and the other reasons the Royer converter together with the lamp inside the system as shown in FIG. 3, provides the system with an asymmetric power waveform of the frequency 55 KHz, unable to meet the requirement of having a crest factor equal to zero. Such a power produces an axial equivalent D. C. electric field in the plasma in the lamp. When the electrode 221 is at a potential relatively higher than the electrode 222, the axial equivalent D. C. electric field forces a steady ion migration of, and a mercury accumulation on the electrode 222. This phenomena means that the mercury component forcing the phosphors to illuminate visible light is decreasing, resulting in luminosity reduction for the lamp 20. Therefore, said mercury accumulation on the cathode, the mercury embedment in the phosphor layer and the formation of an amorphous layer on the phosphor layer by ion bombardment are the three major determinant factors for the life of the lamp which has a minimum requirement of 20,000 hours at 50% luminosity for such application.
Furthermore, the electromagnetic radiation from the A. C. power source interferes with the neighboring electrical equipments; causing a myopia and hazard for the long term user. To limit the field intensity of such a radiation, a Swedish specification TCO91 has been established as follows:
TCO91Electrostatic Potential<±500 voltsMagnetic FieldFrequency Band I 5 Hz~2 kHz≦200 nTrms, measured from30 cm in front of thedisplay and 50 cm aroundthe displayFrequency Band II 2 kHz~400 kHz≦25 nTrms, measured from50 cm around the displayAlternating Electric FieldFrequency Band I 5 Hz~2 kHz≦10 V/mrms, measured from30 cm in front of the displayFrequency Band II 2 kHz~400 kHz≦1.0 V/mrms, measuredfrom 30 cm in front of thedisplay and 50 cm aroundthe display
The present invention is prompted by the intention to solve said problems in said conventional fluorescent lighting systems, yet with low cost solutions.